Motherboards

Easily the most important component in a PC system is the main board or motherboard. Some companies refer to the motherboard
as a system board or planar. The terms motherboard, main board, system board, and planar are interchangeable.
In this chapter, we will examine the different types of motherboards available, as well as those components usually contained
on the motherboard and motherboard interface connectors.

Replacement Motherboards

Some manufacturers go out of their way to make their systems as physically incompatible as possible with any other system.
Then replacement parts, repairs, and upgrades are virtually impossible to find--except, of course, from the original system
manufacturer, at a significantly higher price than the equivalent part would cost to fit a standard PC-compatible system.

For example, if the motherboard in an AT-chassis system dies, you can find any number of replacement boards that will bolt
directly in, with a large choice of processors and clock speeds, at very good prices. If the motherboard dies in a newer IBM,
Compaq, Hewlett-Packard, Packard Bell, Gateway, AST, or other proprietary shaped system, you'll pay for a replacement available
only from the original manufacturer, and you have little or no opportunity to select a board with a faster or better processor
than the one that failed. In other words, upgrading or repairing one of these systems via a motherboard replacement is difficult
and usually not cost-effective.

Knowing What to Look for (Selection Criteria)

You can ask a consultant to make a recommendation for purchases. Making these types of recommendations is one of the most
frequent tasks a consultant performs. Many consultants charge a large fee for this advice. Without guidance, many individuals
don't have any rhyme or reason to their selections and instead base their choices solely on magazine reviews or, even worse,
on some personal bias. To help eliminate this haphazard selection process, here is a simple checklist that will help you select
a system. This list takes into consideration several important system aspects overlooked by most checklists. The goal is to
ensure that the selected system truly is compatible and has a long life of service and upgrades ahead.

It helps to think like an engineer when you make your selection. Consider every aspect and detail of the motherboards in
question. For instance, you should consider any future uses and upgrades. Technical support at a professional (as opposed
to a user) level is extremely important. What support will be provided? Is there documentation, and does it cover everything
else?

In short, a checklist is a good idea. You can use the following check list to evaluate any PC-compatible system. You might
not have to meet every one of these criteria to consider a particular system, but if you miss more than a few of these checks,
consider staying away from that system. The items at the top of the list are the most important, and the items at the bottom
are perhaps of lesser importance. The rest of this chapter discusses in detail the criteria in this checklist:

Processor. The processor is normally the most expensive chip in a system. Because the processor is the brain of
the system, it has a large influence on the performance of the total system. In Chapter 6 - The Microprocessor, you can find
the differences between all types of processors. When you are planning to assemble your own system, you have to realize that
the motherboard also has a large part in the performance of the system. Notice that the processor has to fit on the motherboard
(check for the right socket). Also notice that the operating speed of the CPU clock often depends on the clock speed of the
motherboard.

Motherboard Speed. When you choose a motherboard, first ensure that your processor fits on it. Notice that most
processors run at a multiple of the motherboard speed. For example, Pentium 75 runs at a motherboard speed of 50MHz; Pentium
60, 90, 120, 150, and 180MHz chips run at a 60MHz base motherboard speed; and the Pentium 66, 100, 133, 166, and 200 run at
a 66MHz motherboard speed setting. The Pentium Pro 150, 180, and 200 run at 50, 60, and 66MHz speeds, respectively. All components
on the motherboard (especially cache memory) should be rated to run at the maximum allowable motherboard speed.

Memory. Most motherboards use normal types of memory modules, for example 36-pin SIMMs, 72-pin SIMMs or 168-pin
DIMMs (Dual In-line Memory Modules). Ensure that your memory fits on the motherboard. On some motherboards, the memory modules
must be installed in matched pairs (depends on the bus width of the motherboard and the memory modules). For example, on a
64-bit design Pentium board, the 72-pin SIMMs must be installed in matched pairs, while DIMMs are installed one at a time
(one per 64-bit bank). Carefully consider the total amount of memory that the board supports. The more memory sockets a motherboard
has, the better. Mission-critical systems ideally should use Parity RAM and ensure that the motherboard fully supports parity
checking or even ECC (Error Correcting Code) as well.

Cache Memory. The cache memory is mostly located on the motherboard. Some boards however, have a special socket
to expand the cache memory by placing an extra cache module. Cache memory often is a combination of built-in and on-board
memory. For example: most Pentium Pro processors have a built-in 256K or 512K Level 2 cache, but may also have more Level
2 cache on the motherboard for even better performance.

Bus Type. Be sure the motherboard has the right bus slots for your expansion cards (for example, ISA, MCA or PCI).
Also take a look at the layout of the bus slots to ensure that cards inserted in them will not block access to memory sockets,
or be blocked by other components in the case. Check for extra bus slots to enable expansion of the system in the future.

BIOS. If you build a new system, the motherboard should use an industry-standard BIOS such as those from AMI, Phoenix,
Microid Research, or Award. For easy updating, the BIOS should be of a Flash ROM or EEPROM (Electrically Erasable Programmable
Read Only Memory) design. If you use Plug and Play components, the BIOS should support the PnP specification.

Form Factor. Check for the right form factor of the motherboard and the case, to ensure that the board will fit
in the case. For an explanation of all different types of form factors, see the section 'Motherboard Form Factors' later in
this chapter.

Built-in interfaces. Ideally, a motherboard should contain as many built-in standard controllers and interfaces
as possible. However, when you need a special type of interface, for example a special video card, it's better to choose a
motherboard without a built-in video interface. In most cases the built-in interfaces can be disabled, but not all motherboards
have this feature. Generally there are better choices in external local bus video adapters. Another problem with built-in
interfaces is the driver support. Usually a slot card adapter is more easily supported via standard drivers and is more easily
upgraded as well.

Plug and Play (PnP). If you use Plug and Play components in your system, the motherboard should fully support the
Intel PnP specification. This will allow automatic configuration of PCI adapters as well as PnP ISA adapters.

TIP: Even if a motherboard doesn't list that it's PnP-compatible, it may be. PCI motherboards
are required to be PnP-compatible, as it is a part of the PCI standard.

Power Management. To minimize the power usage of the system, the motherboard should fully support the APM (Advanced
Power Management) and SMM (System Management Mode) protocols that allow for powering down various system components to different
levels of readiness and power consumption.

Motherboard Chipset. There are a lot of different motherboard chipsets on the market. A lot of motherboard features
are based on the chipset. For example, the Intel Triton II (430HX) chipset, allows parity checking. The popular original Intel
Triton (430FX) chipset, along with the newer 430TX and 430VX chipsets, does not support parity-checked memory. For critical
applications where accuracy and data integrity is important, it's recommended that you use a board that support ECC memory
using true parity memory modules.

Documentation. Good technical documentation is a requirement. Documents should include information on any and all
jumpers and switches found on the board, connector pinouts for all connectors, specifications for cache RAM chips, memory
modules, and other plug-in components, and any other applicable technical information. You can acquire separate documentation
from the BIOS manufacturer covering the specific BIOS used in the system, as well as the data books covering the specific
chipset used in the motherboard. Additional data books for any other controller or I/O chips on-board are a bonus, and may
be acquired from the respective chip manufacturers. Another nice thing to have is available online support and documentation
updates, although this should not be accepted in place of good hardcopy manuals.

You may notice that these selection criteria seem fairly strict and may disqualify many motherboards on the market, including
what you already have in your system! These criteria will, however, guarantee you the highest quality motherboard offering
the latest in PC technology that will be upgradable, expandable, and provide good service for many years. It's most recommended
to purchase boards from better-known motherboard manufacturers such as Intel, SuperMicro, Micronics, AMI, Biostar, Tyan, Asus,
and so on. These boards might cost a little more than others that you have never heard of, but there is some safety in the
more well-known brands; that is, the more boards that they sell, the more likely that any problems will have been discovered
by others and solved long before you get yours. Also, if service or support are needed, the larger vendors are more likely
to be around in the long run.

Documentation

As mentioned, extensive documentation is an important factor to consider when you're planning to purchase a motherboard.
Most motherboard manufacturers design their boards around a particular chipset, which actually counts as the bulk of the motherboard
circuitry. There are a number of manufacturers offering chipsets, such as Intel, Opti, VIA, SiS, and others. It's recommended
to obtain the data book or other technical documentation on the chipset directly from the chipset manufacturer.

One of the more common questions of people about a system relates to the BIOS Setup program. People want to know what the
"Advanced Chipset Setup" features mean and what the effects of changing them will be. Often they go to the BIOS manufacturer
thinking that the BIOS documentation will offer help. Usually, however, people find that there is no real coverage of what
the chipset setup features are in the BIOS documentation. You will find this information in the data book provided by the
chipset manufacturer. Although these books are meant to be read by the engineers who design the boards, they contain all the
detailed information about the chipset's features, especially those that might be adjustable. With the chipset data book,
you will have an explanation of all the controls in the Advanced Chipset Setup section of the BIOS Setup program.

Besides the main chipset data books, it's also recommended to collect any data books on the other major chips in the system.
This would include any floppy or IDE controller chips, Super I/O chips, and of course the main processor. You will find an
incredible amount of information on these components in the data books.

CAUTION: Most chipset manufacturers only make a particular chip for a short time, rapidly
superseding it with an improved or changed version. The data books are only available during the time the chip is being manufactured,
so if you wait too long, you will find that such documents may no longer be available. The time to collect documentation on
your new motherboard is now!

ROM BIOS Compatibility

The issue of ROM BIOS compatibility is important. If the BIOS is not compatible, any number of problems can result. Several
reputable companies that produce compatibles have developed their own proprietary ROM BIOS that works just like IBM's. Also,
many of the compatibles' OEMs have designed ROMs that work specifically with additional features in their systems while effectively
masking the effects of these improvements from any software that would "balk" at the differences.

OEMs

Many OEMs (Original Equipment Manufacturers) have developed their own compatible ROMs independently. Companies such as
Compaq and AT&T have developed their own BIOS products which are comparable to those offered by AMI, Phoenix, and others.
These companies also offer upgrades to newer versions that often can offer more features and improvements or fix problems
with the older versions. If you use a system with a proprietary ROM, make sure that it is from a larger company with a track
record and one that will provide updates and fixes as necessary. Ideally, upgrades should be available for download from the
Internet.

Several companies have specialized in the development of a compatible ROM BIOS product. The three major companies that
come to mind in discussing ROM BIOS software are American Megatrends, Inc. (AMI), Award Software, and Phoenix Software. Each
company licenses its ROM BIOS to a motherboard manufacturer so that the manufacturer can worry about the hardware rather than
the software. To obtain one of these ROMs for a motherboard, the OEM must answer many questions about the design of the system
so that the proper BIOS can be either developed or selected from those already designed. Combining a ROM BIOS and a motherboard
is not a haphazard task. No single, generic, compatible ROM exists, either. AMI, Award, Microid Research, and Phoenix ship
to different manufacturers many variations of their BIOS code, each one custom-tailored to that specific system.

AMI

Although AMI customizes the ROM code for a particular system, it does not sell the ROM source code to the OEM. An OEM must
obtain each new release as it becomes available. Because many OEMs don't need or want every new version developed, they might
skip several version changes before licensing a new one.

The AMI BIOS is probably the most used BIOS in PC systems. Newer versions of the AMI BIOS are called Hi-Flex due to the
high flexibility found in the BIOS configuration program. The AMI Hi-Flex BIOS is used in Intel, AMI, and many other manufacturers'
motherboards. One special AMI feature is that it is a third-party BIOS manufacturer that makes its own motherboard.

During powerup, the BIOS ID string is displayed on the lower-left of the screen. This string tells you valuable information
about which BIOS version you have, as well as certain settings which are determined by the built-in setup program.

TIP: A good trick to help you view the BIOS ID string is to shut down and either unplug
your keyboard, or hold down a key as you power back on. This will cause a keyboard error, and the string will remained displayed.

The primary BIOS Identification string (ID String 1) is displayed by any AMI BIOS during the POST (Power On Self-Test)
at the bottom-left corner of the screen, below the copyright message. Two additional BIOS ID strings (ID Strings 2 and 3)
can be displayed by the AMI Hi-Flex BIOS by pressing the Insert key during POST. These additional ID strings display the options
that are installed in the BIOS.

The AMI BIOS has many features, including a built-in setup program activated by pressing the Delete or Esc key in the first
few seconds of booting up your computer. The BIOS will prompt you briefly as to which key to press and when to press it. The
AMI BIOS offers user-definable hard disk types, essential for optimal use of many IDE or ESDI drives. The newer BIOS versions
also support Enhanced IDE drives and will auto-configure the drive parameters.

A unique AMI BIOS feature is that, in addition to the setup, it has a built-in, menu-driven, diagnostics package--essentially
a very limited version of the stand-alone AMIDIAG product. The internal diagnostics are not a replacement for more comprehensive
disk-based programs, but they can help in a pinch. The menu-driven diagnostics does not do extensive memory testing, for example,
and the hard disk low-level formatter works only at the BIOS level rather than at the controller register level. These limitations
often have prevented it from being capable of formatting severely damaged disks.

The AMI BIOS is sold through distributors. However, keep in mind that you cannot buy upgrades and replacements direct from
AMI.

Award

Award is unique among BIOS manufacturers because it sells its BIOS code to the OEM and allows the OEM to customize the
BIOS. Of course, then the BIOS no longer is Award BIOS, but rather a highly customized version. AST uses this approach on
its systems, as do other manufacturers, for total control over the BIOS code, without having to write it from scratch. Although
AMI and Phoenix customize the ROM code for a particular system, they do not sell the ROM's source code to the OEM. Some OEMs
that seem to have developed their own ROM code started with a base of source code licensed to them by Award or some other
company.

The Award BIOS has all the normal features you expect, including a built-in setup program activated by pressing Ctrl+Alt+Esc.
This setup offers user-definable drive types, required in order to fully use IDE or ESDI hard disks.

In all, the Award BIOS is high quality, has minimal compatibility problems, and offers a high level of support.

Phoenix

The Phoenix BIOS for many years has been a standard of compatibility by which others are judged. It was one of the first
third-party companies to legally reverse-engineer the IBM BIOS using a "clean room" approach. In this approach, a group of
engineers studied the IBM BIOS and wrote a specification for how that BIOS should work and what features should be incorporated.
This information then was passed to a second group of engineers who had never seen the IBM BIOS. They could then legally write
a new BIOS to the specifications set forth by the first group. This work would then be unique and not a copy of IBM's BIOS;
however, it would function the same way. This code has been refined over the years and has very few compatibility problems
compared to some of the other BIOS vendors.

The Phoenix BIOS excels in two areas that put it high on the list of recommendations. One is that the POST is excellent.
The BIOS outputs an extensive set of beep codes that can be used to diagnose severe motherboard problems which would prevent
normal operation of the system. In fact, this POST can isolate memory failures in Bank 0 right down to the individual chip
with beep codes alone. The Phoenix BIOS also has an excellent setup program free from unnecessary frills, but that offers
all the features one would expect, such as user-definable drive types, and so on. The built-in setup is activated by pressing
either Ctrl+Alt+S or Ctrl+Alt+Esc, depending on the version of BIOS you have.

The second area in which Phoenix excels is the documentation. Not only are the manuals that you get with the system detailed,
but also Phoenix has written a set of BIOS technical-reference manuals that are a standard in the industry. The set consists
of three books, titled System BIOS for IBM PC/XT/AT Computers and Compatibles, CBIOS for IBM PS/2 Computers and
Compatibles, and ABIOS for IBM PS/2 Computers and Compatibles. Phoenix is one of few vendors who has done extensive
research on the PS/2 BIOS and produced virtually all the ROMs in the PS/2 Micro Channel clones on the market. In addition
to being an excellent reference for the Phoenix BIOS, these books serve as an outstanding overall reference to any company's
IBM-compatible BIOS. Even if you never have a system with a Phoenix BIOS, these books are highly recommended.

Micronics motherboards have always used the Phoenix BIOS, and these motherboards are used in many of the popular name-brand
compatible systems. Phoenix has been one of the largest OEMs of Microsoft MS-DOS. If you have MS-DOS, you also have the Phoenix
OEM version. Phoenix licenses its DOS to other computer manufacturers so long as they use the Phoenix BIOS. Because of its
close relationship with Microsoft, it has had access to the DOS source code, which helps in eliminating compatibility problems.

Unless the ROM BIOS is a truly compatible, custom OEM version such as Compaq's, you might want to install in the system
the ROM BIOS from one of the known quantities, such as AMI, Award, or Phoenix. These companies' products are established as
ROM BIOS standards in the industry, and frequent updates and improvements ensure that a system containing these ROMs will
have a long life of upgrades and service.

Using Correct Speed-Rated Parts

Some compatible vendors use substandard parts in their systems to save money. Because the CPU is one of the most expensive
components on the motherboard, and many motherboards are sold to system assemblers without the CPU installed, it is tempting
for the assembler to install a CPU rated for less than the actual operating speed. A system could be sold as a 100MHz system,
for example, but when you look "under the hood," you may find a CPU rated for only 90MHz. The system does appear to work correctly,
but for how long? If the company that manufactures the CPU chip installed in this system had tested the chip to run reliably
at 100MHz, it would have labeled the part accordingly. After all, the company could sell the chip for more money if it worked
at the higher clock speed.

When a chip is run at a speed higher than it is rated for, it will run hotter than it would normally. This may cause the
chip to overheat occasionally, which would appear as random lockups, glitches, and frustration. It's highly recommended that
you avoid systems whose operation speed exceeds the design of the respective parts.

This practice is easy to fall into because the faster rated chips cost more money, and Intel and other chip manufacturers
usually rate their chips very conservatively. People have taken several 25MHz 486 processors and run them at 33MHz, and they
seemed to work fine. The Pentium 90 chips which are tested seem to run fine at 100MHz. If you purchase a 100MHz system from
a vendor, you would fully expect it to have 100MHz parts, not 90MHz parts running past their rated speed! Many chips have
some form of heat sink on them, which helps to prevent overheating, but which can also sometimes cover up for a "pushed" chip.
If the price is too good to be true, ask before you buy: "Are the parts really manufacturer-rated for the system speed?"

To determine the rated speed of a CPU chip, look at the writing on the chip. Most of the time, the part number will end
in a suffix of -xxx where the xxx is a number indicating the maximum speed. For example, -100 indicates that
the chip is rated for 100MHz operation.

CAUTION: Be careful when running software to detect processor speed. Such programs can only
tell you what speed the chip is currently running at, not what the true rating is. Also ignore the speed indicator lights
on the front of some cases. These digital displays can literally be set via jumpers to read any speed you desire! They have
no true relation to actual system speed.

Motherboard Form Factors

There are several compatible form factors used for motherboards. The form factor refers to the physical dimensions and
size of the board, and dictates what type of case the board will fit into. The types of motherboard form factors generally
available are the following:

Backplane Systems

Full-Size AT

Baby-AT

LPX

ATX

NLX

Backplane Systems

Not all systems have a motherboard in the true sense of the word. In some systems, the components normally found on a motherboard
are located instead on an expansion adapter card plugged into a slot. In these systems, the board with the slots is called
a backplane, rather than a motherboard. Systems using this type of construction are called backplane systems.

Backplane systems come in two main types: passive and active. A passive backplane means the main backplane
board does not contain any circuitry at all except for the bus connectors and maybe some buffer and driver circuits. All the
circuitry found on a conventional motherboard is contained on one or more expansion cards installed in slots on the backplane.
Some backplane systems use a passive design that incorporates the entire system circuitry into a single mothercard. The mothercard
is essentially a complete motherboard that is designed to plug into a slot in the passive backplane. The passive backplane/mothercard
concept allows the entire system to be easily upgraded by changing one or more cards. Because of the expense of the high function
mothercard, this type of system design is rarely found in PC systems. The passive backplane design does enjoy popularity in
industrial systems, which are often rack-mounted. Some high-end file servers also feature this design.

An active backplane means the main backplane board contains bus control and usually other circuitry as well. Most active
backplane systems contain all the circuitry found on a typical motherboard except for the processor complex. The processor
complex is the name of the circuit board that contains the main system processor and any other circuitry directly related
to it, such as clock control, cache, and so forth. The processor complex design allows the user to easily upgrade the system
later to a new processor type by changing one card. In effect, it amounts to a modular motherboard with a replaceable processor
section. Most PC systems that use a backplane design use an active backplane/processor complex. Both IBM and Compaq have used
this type of design in some of their high-end (server class) systems, for example. This allows an easier and generally more
affordable upgrade than the passive backplane/mothercard design since the processor complex board is usually much cheaper
than a mothercard. Unfortunately, because there are no standards for the processor complex interface to the system, these
boards are proprietary and can only be purchased from the system manufacturer. This limited market and availability causes
the prices of these boards to be higher than most complete motherboards from other manufacturers.

The motherboard system design and the backplane system design have both advantages and disadvantages. Most original personal
computers were designed as backplanes in the late 1970s. Apple and IBM shifted the market to the traditional motherboard with
a slot-type design because this type of system generally is cheaper to mass-produce than one with the backplane design. The
theoretical advantage of a backplane system, however, is that you can upgrade it easily to a new processor and new level of
performance by changing a single card. For example, you can upgrade a system's processor just by changing the card. In a motherboard-design
system, you often must change the motherboard itself, a seemingly more formidable task. Unfortunately, the reality of the
situation is that a backplane design is often much more expensive to upgrade, and because the bus remains fixed on the backplane,
the backplane design precludes more comprehensive upgrades that involve adding local bus slots, for example.

Another nail in the coffin of backplane designs is the upgradable processor. Intel has designed all 486 and newer processors
to be upgradable to faster (sometimes called OverDrive) processors in the future by simply swapping (or adding) the
new processor chip. Changing only the processor chip for a faster one is the easiest and generally most cost-effective way
to upgrade without changing the entire motherboard. Because of the limited availability of the processor complex boards or
mothercards, they usually end up being more expensive than a complete new motherboard that uses an industry standard form
factor.

Full-Size AT

The full-size AT motherboard is so named because it matches the original IBM AT motherboard design. This allows for a very
large board of up to 12 inches wide by 13.8 inches deep. The keyboard connector and slot connectors must conform to specific
placement requirements to fit the holes in the case. This type of board will fit into full-size AT or Tower cases only. Because
these motherboards did not fit into the newer Baby-AT or Mini-Tower cases, and because of advances in component miniaturization,
they were no longer being produced when the Baby-AT boards became popular.

Baby-AT

The Baby-AT form factor is essentially the same as the original IBM XT motherboard, with modifications in screw hole positions
to fit into an AT-style case (see Figure 4.1). These motherboards also have specific placement of the keyboard connector and
slot connectors to match the holes in the case. Note that virtually all full-size AT and Baby-AT motherboards use the standard
5-pin DIN type connector for the keyboard. Baby-AT motherboards will fit into every type of case except the Low Profile or
Slimline cases. Figure 4.1 shows the dimensions and layout of a Baby-AT motherboard.

LPX

Another popular form factor used in motherboards is the LPX (and Mini-LPX) form factor. This form factor was first developed
by Western Digital for some of their motherboards. Although they no longer produce PC motherboards, the form factor lives
on and has been duplicated by many other motherboard manufacturers. These are used in most of the Low Profile or Slimline
case systems. It should be noted that systems using LPX boards may have other differences which can cause compatibility problems
similar to those of proprietary systems.

The LPX boards are characterized by several distinctive features. The most noticeable is that the expansion slots are mounted
on a bus riser card that plugs into the motherboard. Expansion cards must plug sideways into the riser card. This sideways
placement allows for the Low Profile case design. Slots are located on one or both sides of the riser card depending on the
system and case design.

Another distinguishing feature of the LPX design is the standard placement of connectors on the back of the board. An LPX
board has a row of connectors for video (VGA 15-pin), parallel (25-pin), two serial ports (9-pin each), and mini-DIN PS/2
style Mouse and Keyboard connectors. All of these connectors are mounted across the rear of the motherboard and protrude through
a slot in the case. Some LPX motherboards may have additional connectors for other internal ports such as Network or SCSI
adapters. Figure 4.2 shows the standard form factors for the LPX and Mini-LPX motherboards.

ATX

The ATX form factor is a combination of the best features of the Baby-AT and LPX motherboard designs, with many new enhancements
and features thrown in. The ATX form factor is essentially a Baby-AT motherboard turned sideways in the chassis, along with
a modified power supply location and connector. The most important thing to know initially about the ATX form factor is that
it is physically incompatible with either the previous Baby-AT or LPX designs. In other words, a different case and power
supply are required to match the ATX motherboard.

The official ATX specification was released by Intel in July 1995, and has been written as an open specification for the
industry. Intel has published detailed specifications so other manufacturers can use the ATX design in their systems.

ATX improves on the Baby-AT and LPX motherboard designs in several major areas:

Built-in double high external I/O connector panel. The rear portion of the motherboard includes a stacked I/O connector
area, which is 6.25 inches wide by 1.75 inches tall. This allows external connectors to be located directly on the board and
negates the need for cables running from internal connectors to the back of the case as with Baby-AT designs.

Single keyed internal power supply connector. This is a boon for the average end user, who always had to worry
about interchanging the Baby-AT power supply connectors and subsequently blowing the motherboard! The ATX specification includes
a single keyed and shrouded power connector that is easy to plug in, and which cannot be installed incorrectly. This connector
also features pins for supplying 3.3v to the motherboard, which means that ATX motherboards will not require built-in voltage
regulators that are susceptible to failure.

Relocated CPU and memory. The CPU and memory modules are relocated so they cannot interfere with any bus expansion
cards, and they can easily be accessed for upgrade without removing any of the installed bus adapters. The CPU and memory
are relocated next to the power supply, which has a single fan blowing air across them, thus eliminating the need for inefficient
and failure-prone CPU cooling fans. There is room for a large passive heat sink above the CPU as well.

Relocated internal I/O connectors. The internal I/O connectors for the floppy and hard disk drives are relocated
to be near the drive bays and out from under the expansion board slot and drive bay areas. This means that internal cables
to the drives can be much shorter, and accessing the connectors will not require card or drive removal.

Improved cooling. The CPU and main memory are cooled directly by the power supply fan, eliminating the need for
separate case or CPU cooling fans. Also, the ATX power supply fan blows into the system chassis, thus pressurizing
it which greatly minimizes dust and dirt intrusion into the system. If desired, an air filter can be easily added to the air
intake vents on the power supply, creating a system that is even more immune to dirt or dust in the environment.

Lower cost to manufacture. The ATX specifications eliminate the need for the rats nest of cables to external port
connectors found on Baby-AT motherboards, eliminates the need for additional CPU or chassis cooling fans, eliminates the need
for on-board 3.3v voltage regulators, uses a single power supply connector, and allows for shorter internal drive cables.
These all conspire to greatly reduce not only the cost of the motherboard, but also significantly reduces the cost of a complete
system including the case and power supply.

Figure 4.3 shows the ATX system layout and chassis features. Notice how the entire motherboard is virtually clear of the
drive bays, and how the devices like CPU, memory, and internal drive connectors are easy to access and do not interfere with
the bus slots. Also notice the power supply orientation and the single power supply fan that blows into the case directly
over the high heat, generating items like the CPU and memory.

The ATX motherboard is basically a Baby-AT design rotated sideways. The expansion slots are now parallel to the shorter
side dimension and do not interfere with the CPU, memory, or I/O connector sockets. In addition to a full-sized ATX layout,
Intel also has specified a mini-ATX design as well, which will fit into the same case. Although the case holes are similar
to the Baby-AT case, cases for the two formats are generally not compatible. The power supplies would require a connector
adapter to be interchangeable, but the basic ATX power supply design is similar to the standard Slimline power supply. The
ATX and mini-ATX motherboard dimensions are shown in Figure 4.4.

NLX

NLX is a Low Profile form factor similar in appearance to LPX, but with a number of improvements designed to allow full
integration of the latest technologies. Whereas the primary limitation of LPX boards includes an inability to handle the physical
size of newer processors, as well as their higher thermal characteristics, the NLX form factor has been designed specifically
to address these problems.

Specific advantages of the NLX form factor include:

Support for newer processor technologies. This is especially important in Pentium II systems because the size of
the Single Edge Contact cartridge this processor uses can limit its use on older motherboard sizes.

Flexibility in the face of rapidly changing processor technologies. Backplane-like flexibility has been built into
the form by allowing a new motherboard to be easily and quickly installed without tearing your entire system to pieces. But
unlike traditional backplane systems, many industry leaders are putting their support behind NLX, including AST, Digital,
Gateway, Hewlett-Packard, IBM, Micron, NEC, and others.

Support for other emerging technologies. This includes Accelerated Graphics Port (AGP) high-performance graphic
solutions, Universal Serial Bus (USB), and tall memory modules and DIMM technology. Furthermore, with the emerging importance
of multimedia applications, connectivity support for such things as video playback, enhanced graphics, and extended audio
have been built into the motherboard. This should represent a good cost savings over expensive daughterboard arrangements,
which have been necessary for many advanced multimedia uses in the past.

Figure 4.5 shows the basic NLX system layout. Notice that, like ATX, the system is clear of the drive bays and other chassis-mounted
components. Also, the motherboard and I/O cards (which, like the LPX form factor, are mounted parallel to the motherboard)
can easily be slid in and out of the side of the chassis, leaving the riser card and other cards in place. The processor itself
can be easily accessed and enjoys greater cooling than in a more closed in layout.

As you can see, the NLX form factor has been designed for maximum flexibility and space efficiency. Even extremely long
I/O cards will fit easily, without fouling on other system components as has been such a problem with Baby-AT form factor
systems.

Motherboard Interface Connectors

There are a variety of different connectors you can find on a motherboard. Most newer motherboards do not only contain
the microprocessor, memory and expansion slots, but also the multi-I/O ports (serial, parallel, harddisk and floppy connectors),
sound system and video adapter. Also Infrared, USB and FireWire connectors can be part of the mainboard.

Generic Connectors

Every motherboard has a standard set of I/O connectors, to connect the power
supply, battery, power LED, keylock and internal speaker. Tables 4.5 through 4.12 contain the pinouts of these generic interface
and I/O connectors.

Table 4.10 Power LED and Keylock Connector

Table 4.11a Speaker Connector Type 1

Table 4.11b Speaker Connector Type 2

Pin

Signal Name

1

Ground

2

Reserved / Access key

3

Board-Mounted Speaker *

4

Speaker Output

Some boards have a board mounted piezo speaker. It is enabled by placing a jumper over pins 3 and 4, which routes the
speaker output to the board mounted speaker. Removing the jumper allows a conventional speaker to be plugged in.

Table 4.12 Microprocessor Fan Power Connector

Pin

Signal Name

1

Ground

2

+12v

3

Ground / Sense tachometer

CAUTION: Do not place a jumper on this connector; serious board damage will result if the
+12v is shorted to ground.

Motherboard Mouse (PS/2)

Newer motherboards also can have a motherboard mouse (PS/2) connector.
The motherboard mouse port normally uses either a 5-pin (one of two types) or a 10-pin connector. Tables 4.13a, 4.13b and
4.13c show the pinouts of these three different motherboard mouse connectors.

Table 4.13a Motherboard Mouse 5-Pin-Header Connector Type 1

Pin

Signal Name

1

+5v

2

Reserved

3

Ground

4

CLK

5

Data

Table 4.13b Motherboard Mouse 5-Pin-Header Connector Type 2

Pin

Signal Name

1

Data

2

Reserved

3

Ground

4

+5v

5

CLK

Table 4.13c Motherboard Mouse 10-Pin-Header Connector

Pin

Signal Name

Pin

Signal Name

1

Ground

6

Reserved

2

Reserved

7

Data

3

Reserved

8

+5v

4

Reserved

9

Reserved

5

Access key

10

Reserved

NOTE: If you don't know which of these connectors is used, check the motherboard documentation.
For the PS/2 bracket, usually the wires are colored as follows: red = Data, green = Ground, yellow = +5v and blue = CLK.

Infrared Data (IrDA)

The infrared I/O port can use either a 4-pin, 5-pin, 6-pin or 7-pin connector. Note that only the 5-pin connector is the
official IrDA standard. Tables 4.14a through 4.14d show the pinouts of the four different infrared connectors.

Table 4.14a Infrared 4-Pin-Header Connector

Pin

Signal Name

1

+5v

2

IrRX

3

Ground

4

IrTX

Table 4.14b Standard Infrared Data (IrDA) 5-Pin-Header Connector

Pin

Signal Name

1

+5v

2

Access key

3

IrRX

4

Ground

5

IrTX

Table 4.14c Infrared 6-Pin-Header Connector

Pin

Signal Name

1

+5v

2

Access key

3

IrRX

4

Ground

5

IrTX

6

Reserved

Table 4.14d Infrared 7-Pin-Header Connector

Pin

Signal Name

1

IrRX

2

Ground

3

IrTX

4

+5v

5

IrRXH

6

+5v

7

Ground

Video Feature Connectors

When the VGA chipset is embedded in the motherboard, in most cases one type of feature connector is available. This is
an additional connector that is used to connect the video chipset to other video devices, such as 3D accelerators, MPEG decoders
and video capture cards. The reason that these connectors are used is that they permit the direct transfer of video information
from these devices to the video card, without having to use the main system bus. Even high-performance local buses can get
slowed down when trying to deal with the enormous amount of information that, for example, a full-motion video stream represents.

The VESA Feature Connector (VFC)

The VESA Feature Connector (VFC) is the first standard in video connectors. The VFC interface is 8-bit wide, resulting
in a maximum resolution of 640x480 with a maximum of 256 colors (standard VGA). Table 4.15 shows the pinouts of the 26-pin-header
VESA Feature Connector.

Table 4.15 VESA Feature Connector

Pin

Signal Name

Pin

Signal Name

1

DAC Pixel Data 0 / PB

14

Ground

2

DAC Pixel Data 1 / PG

15

Ground

3

DAC Pixel Data 2 / PR

16

Ground

4

DAC Pixel Data 3 / PI

17

Select Internal Video

5

DAC Pixel Data 4 / SB

18

Select Internal Sync

6

DAC Pixel Data 5 / SG

19

Select Internal Dot Clock

7

DAC Pixel Data 6 / SR

20

Reserved

8

DAC Pixel Data 7 / SI

21

Ground

9

DAC Clock

22

Ground

10

DAC Blanking

23

Ground

11

Horizontal Sync

24

Ground

12

Vertical Sync

25

Reserved

13

Ground

26

Reserved

The VESA Advanced Feature Connector (VAFC)

The VESA Advanced Feature Connector (VAFC) is developed as an extension to the VESA Feature Connector. Version 1.0
of the VAFC was released in December 1995. The VAFC interface widens the port from 8 bits to 16 or 32, and provides improved
signaling for more reliability. The maximum clock rate is 37.5MHz, resulting in a maximum data transfer rate of 150M/sec in
32-bit mode. An 80-pin connector is used. Table 4.16 shows the pinouts of the 80-pin VESA Advanced Feature Connector.

Table 4.16 VESA Advanced Feature Connector

Pin

Signal Name

Pin

Signal Name

1

RSRV 0

41

Ground

2

RSRV 1

42

Ground

3

GENCLK

43

Ground

4

OFFSET 0

44

Ground

5

OFFSET 1

45

Ground

6

FSTAT

46

Ground

7

VRDY

47

Ground

8

GRDY

48

Ground

9

DAC Blanking

49

Ground

10

Vertical Sync

50

Ground

11

Horizontal Sync

51

Ground

12

EGEN

52

Ground

13

VCLK

53

Ground

14

RSRV 2

54

Ground

15

DAC Clock

55

Ground

16

External Video Select

56

Ground

17

DAC Pixel Data 0

57

DAC Pixel Data 1

18

Ground

58

DAC Pixel Data 2

19

DAC Pixel Data 3

59

Ground

20

DAC Pixel Data 4

60

DAC Pixel Data 5

21

Ground

61

DAC Pixel Data 6

22

DAC Pixel Data 7

62

Ground

23

DAC Pixel Data 8

63

DAC Pixel Data 9

24

Ground

64

DAC Pixel Data 10

25

DAC Pixel Data 11

65

Ground

26

DAC Pixel Data 12

66

DAC Pixel Data 13

27

Ground

67

DAC Pixel Data 14

28

DAC Pixel Data 15

68

Ground

29

DAC Pixel Data 16

69

DAC Pixel Data 17

30

Ground

70

DAC Pixel Data 18

31

DAC Pixel Data 19

71

Ground

32

DAC Pixel Data 20

72

DAC Pixel Data 21

33

Ground

73

DAC Pixel Data 22

34

DAC Pixel Data 23

74

Ground

35

DAC Pixel Data 24

75

DAC Pixel Data 25

36

Ground

76

DAC Pixel Data 26

37

DAC Pixel Data 27

77

Ground

38

DAC Pixel Data 28

78

DAC Pixel Data 29

39

Ground

79

DAC Pixel Data 30

40

DAC Pixel Data 31

80

Ground

The VESA Media Channel (VMC)

The VESA Media Channel (VMC) is a new developed industry bus standard, dedicated specifically to the problem of
transferring raw, uncompressed digital video and audio data through the PC, without incurring performance overheads. VMC operates
at a clock speed of 33MHz, resulting in a maximum data transfer rate of 132M/sec. VMC supports 8-bit, 16-bit and 32-bit devices
simultaneously, and handles up to 15 data streams at the time without affecting performance. The VESA Media Channel provides
the option for a 68-pin multi-drop cable, allowing multiple devices to be combined in a modular fashion. Table 4.17 shows
the pinouts of the 68-pin VESA Media Channel Connector.

Table 4.17 VESA Media Channel Connector

Pin

Signal Name

Pin

Signal Name

1

SA

35

EVST(0)

2

EVST

36

Ground

3

BS(0)

37

BS(1)

4

Ground

38

SNRDY

5

CONTROL

39

Ground

6

RESET

40

Ground

7

CLOCK

41

Ground

8

Reserved

42

Ground

9

MASK 0

43

MASK 1

10

Ground

44

DAC Pixel Data 0

11

DAC Pixel Data 1

45

Ground

12

DAC Pixel Data 2

46

DAC Pixel Data 3

13

Ground

47

DAC Pixel Data 4

14

DAC Pixel Data 5

48

Ground

15

DAC Pixel Data 6

49

DAC Pixel Data 7

16

Ground

50

DAC Pixel Data 8

17

DAC Pixel Data 9

51

Ground

18

DAC Pixel Data 10

52

DAC Pixel Data 11

19

Ground

53

DAC Pixel Data 12

20

DAC Pixel Data 13

54

Ground

21

DAC Pixel Data 14

55

DAC Pixel Data 15

22

Ground

56

DAC Pixel Data 16

23

DAC Pixel Data 17

57

Ground

24

DAC Pixel Data 18

58

DAC Pixel Data 19

25

Ground

59

DAC Pixel Data 20

26

DAC Pixel Data 21

60

Ground

27

DAC Pixel Data 22

61

DAC Pixel Data 23

28

Ground

62

DAC Pixel Data 24

29

DAC Pixel Data 25

63

Ground

30

DAC Pixel Data 26

64

DAC Pixel Data 27

31

Ground

65

DAC Pixel Data 28

32

DAC Pixel Data 29

66

Ground

33

DAC Pixel Data 30

67

DAC Pixel Data 31

34

Ground

68

SB

USB Motherboard Connector

Newer motherboards often have embedded USB chipsets. When the USB connectors are not soldered onto the motherboard, a separate
USB bracket is used. Table 4.18a, 18b and 18c show the pinouts of the different types of USB motherboard connectors for the
USB bracket. Note that in some cases the two separate connectors are joined to one 8-pin or 10-pin connector.

Table 4.18a Separate 4-pin USB Motherboard Connectors

Pin

Signal Name

Pin

Signal Name

1

+5v

1

+5v

2

Data -

2

Data -

3

Data +

3

Data +

4

Ground

4

Ground

Table 4.18b Separate 5-pin USB Motherboard Connectors Type 1

Pin

Signal Name

Pin

Signal Name

1

+5v

1

+5v

2

Data -

2

Data -

3

Data +

3

Data +

4

Ground

4

Ground

5

Ground

5

Access Key

Table 4.18c Separate 5-pin USB Motherboard Connectors Type 2

Pin

Signal Name

Pin

Signal Name

1

+5v

1

+5v

2

Reserved / Access Key

2

Reserved / Access Key

3

Data -

3

Data -

4

Data +

4

Data +

5

Ground

5

Ground

NOTE: If you don't know which of these connectors is used, check the motherboard documentation.
For the USB bracket, usually the wires are colored as follows: red = +5v, black = Ground, white = Data - and green = Data
+.

Motherboard CMOS RAM Addresses

Table 4.19 shows the information maintained in the 64-byte standard CMOS RAM module. This information controls the configuration
of the system and is read and written by the system Setup program.

In the original AT system, a Motorola 146818 chip was used. Newer systems incorporate the CMOS into the chipset, Super
I/O chip, or use a special battery and NVRAM (Non-Volatile RAM) module from companies like Dallas or Benchmarq. The standard
format of the information stored in the CMOS RAM is shown in Table 4.19.

Table 4.19 AT CMOS RAM Addresses

Offset Hex

Offset Dec

Field Size

Function

00h

0

1 byte

Current second in
binary coded decimal (BCD)

01h

1

1 byte

Alarm second in BCD

02h

2

1 byte

Current minute in
BCD

03h

3

1 byte

Alarm minute in BCD

04h

4

1 byte

Current hour in BCD

05h

5

1 byte

Alarm hour in BCD

06h

6

1 byte

Current day of week
in BCD

07h

7

1 byte

Current day in BCD

08h

8

1 byte

Current month in
BCD

09h

9

1 byte

Current year in BCD

0Ah

10

1 byte

Status register A

Bit 7 = Update in
progress

0 = Date and time can be read

1 = Time update in progress

Bits 6-4 = Time frequency
divider

010 = 32.768KHz

Bits 3-0 = Rate selection
frequency

0110 = 1.024KHz square wave
frequency

0Bh

11

1 byte

Status register B

Bit 7 = Clock update
cycle

0 = Update normally

1 = Abort update in progress

Bit 6 = Periodic
interrupt

0 = Disable interrupt (default)

1 = Enable interrupt

Bit 5 = Alarm interrupt

0 = Disable interrupt (default)

0 = Disable interrupt (default)

1 = Enable interrupt

Bit 4 = Update-ended interrupt

0 = Disable interrupt (default)

1 = Enable interrupt

Bit 3 = Status register
A square wave frequency

0 = Disable square wave (default)

1 = Enable square wave

Bit 2 = Date format

0 = Calendar in BCD format
(default)

1 = Calendar in binary format

Bit 1 = 24-hour clock

0 = 24-hour mode (default)

1 = 12-hour mode

Bit 0 = Daylight
Savings Time

0 = Disable Daylight Savings
(default)

1 = Enable Daylight Savings

0Ch

12

1 byte

Status register C

Bit 7 = IRQF flag

Bit 6 = PF flag

Bit 5 = AF flag

Bit 4 = UF flag

Bits 3-0 = Reserved

0Dh

13

1 byte

Status register D

Bit 7 = Valid CMOS
RAM bit

0 = CMOS battery dead

1 = CMOS battery power good

Bits 6-0 = Reserved

0Eh

14

1 byte

Diagnostic status

Bit 7 = Real-time
clock power status

0 = CMOS has not lost power

1 = CMOS has lost power

Bit 6 = CMOS checksum
status

0 = Checksum is good

1 = Checksum is bad

Bit 5 = POST configuration
information status

0 = Configuration information
is valid

1 = Configuration information
is invalid

Bit 4 = Memory size
compare during POST

0 = POST memory equals

configuration

1 = POST memory not equal to

configuration

Bit 3 = Fixed disk/adapter
initialization

0 = Initialization good

1 = Initialization failed

Bit 2 = CMOS time
status indicator

0 = Time is valid

1 = Time is Invalid

Bits 1-0 = Reserved

0Fh

15

1 byte

Shutdown code

00h = Power on or
soft reset

01h = Memory size
pass

02h = Memory test
pass

03h = Memory test
fail

04h = POST end; boot
system

05h = JMP double
word pointer with EOI

06h = Protected mode
tests pass

07h = Protected mode
tests fail

07h = Protected mode
tests fail

08h = Memory size
fail

09h = Int 15h block
move

0Ah = JMP double
word pointer without

EOI

0Bh = used by 80386

10h

16

1 byte

Floppy disk drive
types

Bits 7-4 = Drive
0 type

Bits 3-0 = Drive
1 type

0000 = None

0001 = 360K

0010 = 1.2M

0011 = 720K

0100 = 1.44M

11h

17

1 byte

Reserved

12h

18

1 byte

Hard disk types

Bits 7-4 = Hard disk
0 type (0-15)

Bits 3-0 = Hard disk
1 type (0-15)

13h

19

1 byte

Reserved

14h

20

1 byte

Installed equipment

Bits 7-6 = Number
of floppy disk drives

00 = 1 floppy disk drive

01 = 2 floppy disk drives

Bits 5-4 = Primary
display

00 = Use display adapter BIOS

01 = CGA 40-column

10 = CGA 80-column

11 = Monochrome Display Adapter

Bits 3-2 = Reserved

Bit 1 = Math coprocessor
present

Bit 0 = Floppy disk
drive present

15h

21

1 byte

Base memory low-order
byte

16h

22

1 byte

Base memory high-order
byte

17h

23

1 byte

Extended memory low-order
byte

18h

24

1 byte

Extended memory high-order
byte

19h

25

1 byte

Hard Disk 0 Extended
Type (0-255)

1Ah

26

1 byte

Hard Disk 1 Extended
Type (0-255)

1Bh

27

9 bytes

Reserved

2Eh

46

1 byte

CMOS checksum high-order
byte

2Fh

47

1 byte

CMOS checksum low-order
byte

30h

48

1 byte

Actual extended memory
low-order byte

31h

49

1 byte

Actual extended memory
high-order byte

32h

50

1 byte

Date century in BCD

33h

51

1 byte

POST information
flag

Bit 7 = Top 128K
base memory status

0 = Top 128K base memory not
installed

1 = Top 128K base memory installed

Bit 6 = Setup program
flag

0 = Normal (default)

1 = Put out first user message

Bits 5-0 = Reserved

34h

52

2 bytes

Reserved

Table 4.20 shows the values that may be stored by your system BIOS in a special CMOS byte called the diagnostics status
byte. By examining this location with a diagnostics program, you can determine whether your system has set trouble
codes, which indicate that a problem has occurred previously.

Table 4.20 CMOS RAM (AT and PS/2) Diagnostic Status Byte Codes

Bit Number

7 6 5 4 3 2 1 0

Hex

Function

1 . . . . . . .

80

Real-time clock (RTC) chip
lost power

. 1 . . . . . .

40

CMOS RAM checksum is bad

. . 1 . . . . .

20

Invalid configuration information
found at POST

. . . 1 . . . .

10

Memory size compare error at
POST

. . . . 1 . . .

08

Fixed disk or adapter failed
initialization

. . . . . 1 . .

04

Real-time clock (RTC) time
found invalid

. . . . . . 1 .

02

Adapters do not match configuration

. . . . . . . 1

01

Time-out reading an adapter
ID

. . . . . . . .

00

No errors found (Normal)

If the Diagnostic status byte is any value other than zero, you will normally get a CMOS configuration error on bootup.
These types of errors can be cleared by re-running the Setup program.